By Chris McCahill
Traffic engineers across the U.S. are accustomed to measuring road performance in terms of “level of service,” or LOS. Recognizing its unintended consequences, many transportation professionals and advocates have urged the industry to replace it with something better. However, LOS has become so ingrained in many processes that there is probably no single alternative to its use.
LOS is a straightforward concept outlined in the Highway Capacity Manual. Free-flowing roads get an A, and roads that have reached their capacity get an F. For signalized intersections, it is based on the average delay (a delay of more than 80 seconds is an F).
Now, there is a growing push to abandon LOS and replace it with a measure of vehicle miles traveled, or VMT. This idea originates mainly from California’s SB 743, which disqualified LOS as a measure of air quality impacts from transportation, and eventually replaced it with VMT. Under the rule, the state recognized that efforts to improve LOS typically lead to more driving, discourage multimodal improvements, and make urban development more difficult. In contrast, VMT is meant to encourage transportation investments and land use patterns that facilitate less driving through shorter trips and greener modes.
Todd Litman, Ousama Shebeeb, and Ron Milam explain in a recent issue of ITE Journal:
A new paradigm is changing the way we define transportation problems and evaluate potential solutions. The old “mobility-based” paradigm assumed that our goal is to maximize travel speed. The new “accessibility-based” paradigm recognizes that the goal of most travel activity is access to services and activities such as shopping, education, and work.
They add:
To guide decisions to support vehicle travel reduction targets, practitioners can change transportation performance indicators from LOS—which assumes that the goal is to maximize vehicle travel speeds, to VMT—which assumes that the goal is to reduce the amount of travel required to satisfy accessibility demands.
A challenge to making this shift, however, is that the environmental review process (as it is used in California) is just one way that road performance measures like LOS are used. To adjust the timing of a traffic signal or estimate the traffic impacts of a new development, for example, most engineers still rely on some measure of traffic delay.
Researchers at UC Davis recently published a study looking at the implementation of SB 743 across 274 local jurisdictions in California. They explain, “all the jurisdictions for which we found information about their use of LOS continue to employ the metric for planning and project-level review outside of CEQA,” the state’s environmental quality act. “A couple of interviewees mentioned that they continue using LOS in traffic circulation and network performance evaluation in their congestion management plans.” An earlier survey found almost half of local officials in California still planned to use LOS for determining traffic impact fees to be paid by developers.
Given its many uses, the answer may not be to abandon LOS entirely, but to arm engineers with better tools for evaluating and addressing traffic problems in different contexts. LOS lets engineers “take the temperature” of a road or intersection. But, like a doctor treating a patient, the temperature alone does not point them to the right treatment. An engineer might also want to evaluate multimodal accessibility and balance the needs of non-drivers. More importantly, they may need to address congestion through strategies for managing travel demand, rather than by increasing the capacity of the road.
The same thinking often applies to development projects. When evaluating the traffic impacts of new developments, local governments typically assess whether a project will push the LOS of nearby road or intersections beyond a certain threshold. However, some communities now focus on reducing the traffic a development generates in the first place. This approach, which we call “modern mitigation,” includes limiting parking, charging for it, offering bus passes, and investing in walking, biking, and other transportation options. This not only reduces negative impacts but also makes infill development more feasible.
Our mobility-based paradigm, and related measures like LOS, have failed to make transportation as safe or efficient as it should be, and they have made our environmental challenges worse. Measures like VMT can be useful alternatives in certain cases, but they are part of a larger paradigm shift, and a broader range of tools needed to make that shift.
Photo Credit: Aayush Srivastava via Pexels, unmodified. License.